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    <title>Document</title>
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<body>
    文本框：<input type="text">用于输入文本<br>
    口令框：<input type="password">用于输入口令<br>
    单选框：<input type="radio">用于选择一项<br>
    复选框：<input type="checkbox">用于多项选择<br>
    下拉框：<select>用于选择一项</select>
           <select>a</select>
           <select>b</select>
           <select>c</select>
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